In general, a natural rubber coagulum produced from a cup lump contains a moisture in a standing step thereof. Accordingly, bacteria and microorganisms contained in the coagulum are proliferated and activated, and proteins and lipids which are non-rubber components in the natural rubber coagulum are decomposed thereby. Also, a polyisoprene chain itself of natural rubber is influenced thereby as well and decreased in a molecular weight in a standing step.
Reduction in a molecular weight of a natural rubber coagulum containing a moisture in a standing step brings about marked reduction in physical properties such as a modulus and a tensile stress in the rubber composition thereof. Also, a change in a natural rubber coagulum with the passage of time in a standing step is dispersed to a large extent, and the targeted physical properties are not obtained in the rubber composition thereof in a certain case. The variation in a change with the passage of time which is referred to in the above case is variation in a change with the passage of time in the natural rubber coagulum itself in a certain period and variation originating in a difference of a standing period of the natural rubber coagulum.
Known are natural rubber latices to which preservatives, fungicides and bacteriostatic agents are added for the purpose of inhibiting proteins and lipids which are non-rubber components in a natural rubber latex from being decomposed by microorganisms in order to obtain a highly stable natural rubber latex which is not coagulated even after left standing for a long period in a nearly neutral pH region from a weak acidity to a weak alkalinity (refer to patent documents 1 and 2). The purposes of the above procedures are to secure stability of a natural rubber latex in a standing step and/or simplification of treatments in subsequent steps.
In recent years, a natural rubber coagulum originating in a cup lump is increased in use thereof. In the production process of the natural rubber described above, natural rubber latices gathered in farms and plantations are coagulated, and the coagulation products are delivered to processing plants in the form of natural rubber coagulums in a principal case. The natural rubber coagulums are left standing for various periods, and reduction and variation in the physical properties of the natural rubber described above are caused by a difference in time of standing them, and that is a large problem. Further, a difference in the physical properties of the natural rubber coagulum which originates in a storing condition thereof in a standing step is a problem as well, and it is desired to stabilize the physical properties of the natural rubber coagulum and inhibit the physical properties thereof from being dispersed due to a difference in a standing period.